Evidence of Silicene in Honeycomb Structures of Silicon on Ag(111)
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Letter pubs.acs.org/NanoLett Evidence of Silicene in Honeycomb Structures of Silicon on Ag(111) † † † ‡ † † † † Baojie Feng, Zijing Ding, Sheng Meng, Yugui Yao, , Xiaoyue He, Peng Cheng, Lan Chen,*, † and Kehui Wu*, † Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡ School of Physics, Beijing Institute of Technology, Beijing 100081, China ABSTRACT: In the search for evidence of silicene, a two- dimensional honeycomb lattice of silicon, it is important to obtain a complete picture for the evolution of Si structures on Ag(111), which is believed to be the most suitable substrate for growth of silicene so far. In this work we report the finding and evolution of several monolayer superstructures of silicon on Ag(111), depend- ing on the coverage and temperature. Combined with first- principles calculations, the detailed structures of these phases have been illuminated. These structures were found to share common building blocks of silicon rings, and they evolve from a fragment of silicene to a complete monolayer silicene and multilayer silicene. Our results elucidate how silicene forms on Ag(111) surface and provides methods to synthesize high-quality and large- scale silicene. KEYWORDS: Silicene, Ag(111), scannning tunneling microscopy, molecular beam epitaxy, first-principles calculation ith the development of the semiconductor industry of silicene and optimizing the preparation procedure for W toward a smaller scale, the rich quantum phenomena in growing high-quality silicene films, it is important to build a low-dimensional systems may lead to new concepts and complete understanding of the formation mechanism and ground-breaking applications. In the past decade graphene growth dynamics of possible silicon structures on Ag(111), has emerged as a low-dimensional system for both fundamental which is currently believed to be the best substrate for growing research and novel applications including electronic devices, silicene. 1−4 energy storage, and transparent protection layer. Inspired by In this Letter, we present a systematic study of the self- the fruitful results based on graphene, recently a lot of interest organized superstructures formed by submonolayer silicon 5−7 has been drawn to group IV (Si, Ge) analogs of graphene. It grown on Ag(111), by STM and scanning tunneling spectros- has been theoretically shown that silicene, with Si atoms packed copy (STS). We found that, depending on the substrate in a honeycomb lattice, like graphene, is a new massless Dirac temperature and silicon coverage, several monolayer super- Fermion system.5,8 Compared with that of graphene, the − structures can form on Ag(111). These superstructures are stronger spin orbit coupling in silicene may lead to a distinct from any known surface structures of bulk silicon and detectable quantum spin Hall effect (QSHE) and other 8−11 are characterized by honeycomb building blocks and structures. attractive properties. The compatibility of silicene with At sufficiently high temperature and Si coverage, monolayer silicon-based nanotechnology makes this material particularly and multilayer silicene films were grown. Combined with first- interesting for device applications. principles calculations, the structural models of these phases are As the theoretical studies on silicene are rapidly increasing, proposed, and their evolution with temperature and Si coverage the major challenge in this field is now the preparation of high- quality silicene films. However, to date, there is still no solid is discussed. Our work provides a complete understanding of evidence for the observation of a silicene film. There have been the structure evolution of Si on Ag(111), which is desirable for a few works on the formation of silicene nanoribbons on fabrication of high-quality silicene and exploring its novel Ag(110) with graphene-like electronic signature.12,13 The only physics and applications. published work on the preparation of silicene-like sheets was Experiments and Methods. Experiments were carried out reported by Lalmi et al. on Ag(111).14 They showed scanning in a home-built low-temperature STM with a base pressure of 5 × −11 tunneling microscopy (STM) images of a honeycomb 10 Torr. Single-crystal Ag(111) sample was cleaned by monolayer structure that resembles monolayer graphene cycles of argon ion sputtering and annealing. Silicon was ≈ structure. However, in their experiment the observed lattice evaporated from a heated wafer ( 1200 K) onto the preheated − constant was about 17% smaller than the theoretically proposed substrate. The deposition rate of silicon was kept at 0.08 0.1 model or the value of bulk silicon. Such a huge compression of the lattice is rather unlikely to be induced by the strain between Received: March 17, 2012 the film and the substrate. Their results therefore remain to be Revised: May 29, 2012 confirmed and understood. For the purpose of finding evidence Published: June 1, 2012 © 2012 American Chemical Society 3507 dx.doi.org/10.1021/nl301047g | Nano Lett. 2012, 12, 3507−3511 Nano Letters Letter ML/min (here 1 monolayer refers to the atomic density of a ideal silicene sheet). The STS data were acquired using a lock- in amplifier by applying a small sinusoidal modulation to the tip bias voltage (typically 10 mV at 676 Hz). All our STM experiments were carried out at 77 K. First-principles calculations were performed within the framework of density functional theory (DFT) using projected augmented wave (PAW)15,16 pseudopotentials and the Perdew−Burke−Ernzerholf (PBE)17 form for exchange− correlation functional, as implemented in Vienna ab initio simulation package (VASP).18 During calculations, the structures were relaxed without any symmetry constraints using a plane-wave energy cutoff of 250 eV. The convergence of energy is set to 1.0 × 10−4 eV. The relaxation process continues until forces are below 0.01 eV/Å. Results and Discussions. Silicon atoms deposited on Ag(111) tend to form clusters or other disordered structures when the substrate temperature is below 400 K during growth (data not shown here). As substrate temperature increases to 420 K, two ordered phases form, as shown in Figure 1. The less ordered phase consists of close-packed protrusions (labeled T), and the highly ordered phase exhibits honeycomb structures (labeled H). The two phases can coexist on the surface within a large coverage range, from 0.5 to 0.9 ML (Figure 1a,b, respectively). The coexistence of phases H and T indicates that these two phases have quite similar formation energy and stability. One can therefore expect similarity and relations between the atomic structure of these two phases. Indeed, the high-resolution STM images in Figure 1c,d show that every big bright protrusion in both phase T and H is indeed composed of three smaller spots that we refer to as a “trimer”, although in phase H the trimers are perfectly ordered, while in phase T they exhibit some irregularity and distortion when looked at closely. The two Figure 1. (a) STM image (Vtip = 1.2 V) of 0.5 ML silicon atoms phases share the same periodicity of 1.18 nm, therefore the deposited on Ag(111) surface at substrate temperature of 420 K. The density of trimers in phase H is just twice of that in phase T. areas with phase T were marked by “T”, while the areas with phase H ff − We further performed STS measurements for the two phases (with di erent rotation angles) were marked by H1 H3, respectively. − (Figure 1e). The dI/dV curves exhibit very similar features of (b) STM image (Vtip = 1.5 V) of 0.9 ML silicon atoms deposited on electronic density of states (DOS), which strongly implies that Ag(111) surface at substrate temperature of 440 K. The areas of phase − the two phases may share some common building blocks in T and H are labeled. Notably H1 H3 mark areas with phase H in different orientations. (c,d) High-resolution STM images (8.5 × 8.5 their atomic structures. In fact, in Figure 1c, there is a 2 − noticeable point showing a corner hole and six protrusions nm , Vtip = 1.0 V) showing the atomic structure of phase T and H, respectively. The blue rhombuses in (c,d) indicate the unit cells of two surrounding it a characteristic signature of formation of phase phases. (e) dI/dV spectra taken at areas of phase T (red) and H H. Moreover, we notice that phase T prefers to form at a lower (black), respectively. The curves have been shifted vertically for clarity. Si coverage and a slightly lower temperature as compared with phase H. With the increase of substrate temperature and coverage, phase T decreases in percentage and eventually of Si equals exactly four times the lattice constant of Ag. If one disappears completely at 460 K. Meanwhile, phase H can assume the observed H phase to be the theoretically proposed spread over the surface if the coverage is sufficiently high. This silicene, it is possible to obtain a 3 × 3 superstructure by placing means that phase H is more stable than phase T, and phase T the silicene lattice in parallel with the 1 × 1 Ag lattice. However, can be regarded as a precursor state of phase H. the crucial point in such moirépattern models is that the We now face a direct question whether these two phases, periodicity of the superstructure, or essentially the moiré especially the well-ordered H phase, are the theoretically pattern, is strictly linked with the relative orientations of the proposed silicene. The phase T can be excluded first due to the two overlapping lattices. If one obtain a 3 × 3 moirépattern in significantly lower density of Si than that of phase H. We notice one orientation, it will be impossible to observe the same that the periodicity of 1.18 nm is almost exactly four times the pattern in another inequivalent crystallographic orientation.